Question

A plant with orange, spotted flowers was grown in the greenhouse from a seed collected in the wild. The plant was self-pollinated and gave rise to the following progeny: 88 orange with spots, 34 yellow with spots, 32 orange with no spots, and 8 yellow with no spots. What can you conclude about the dominance relationships of the alleles responsible for the spotted and unspotted phenotypes? What can you conclude about the genotype of the original plant that had orange, spotted flowers?

Short answer

The dominance relationship of the alleles responsible for spotted and unspotted phenotypes is that the spotted phenotype is dominant. The original plant, which had orange-spotted flowers, has a genotype of OY Ss, indicating incomplete dominance between orange and yellow alleles. The observed progeny ratios 10:7 for orange to yellow and 3:1 for spotted to unspotted support these conclusions.

Step-by-step solution

List down the observed progeny and phenotypes

The information given about the progeny is as follows: 1. 88 orange with spots 2. 34 yellow with spots 3. 32 orange with no spots 4. 8 yellow with no spots

Identify the ratios from the given information

From the observed progeny, we can see that the ratio of orange to yellow is 120:42 (88+32:34+8) which simplifies to 10:7. Similarly, the ratio of spotted to no spots is 122:40 (88+34:32+8) simplifying to approximately 3:1.

Determine the dominance relationships

From the spotted to unspotted ratio of approximately 3:1, we can infer that the allele for the spotted phenotype is dominant. Let's represent the dominant allele for the spotted phenotype with 'S' and the recessive allele with 's'. Since the orange to yellow ratio is not a simple Mendelian ratio, we can assume that there is incomplete dominance between orange and yellow alleles. Let's represent the alleles for flower color with 'O' for orange and 'Y' for yellow.

Determine the genotype of the original plant

Since the original plant had orange-spotted flowers and was self-pollinated, the genotype must be heterozygote, because if it was homozygote, we would not have offspring with different flower colors or phenotypes. Thus, the genotype of the original plant can be represented as OY Ss.

Use the Punnett square to verify our conclusions

We create a Punnett square with the genotype OY Ss crossed with itself. | | O S | O s | Y S | Y s | |---|---|---|---|---| | O S | OO SS | OO Ss | OY SS | OY Ss | | O s | OO Ss | OO ss | OY Ss | OY ss | | Y S | OY SS | OY Ss | YY SS | YY Ss | | Y s | OY Ss | OY ss | YY Ss | YY ss | From the Punnett square, we have the following progeny: 1. 25% OO SS - Orange with spots 2. 50% OO Ss - Orange with spots 3. 12.5% OY SS - Orange with spots 4. 12.5% YY SS - Yellow with spots 5. 25% OY Ss - Orange with spots 6. 25% OY ss - Orange with no spots 7. 12.5% YY Ss - Yellow with no spots 8. 12.5% YY ss - Yellow with no spots From this Punnett square, we can see the orange to yellow ratio of approximately 10:7 and a spotted to unspotted phenotype ratio of 3:1. #Conclusion#: From the self-pollination of the given plant, we can conclude that the alleles responsible for the spotted phenotype have a dominance relationship, with the spotted phenotype being dominant. The genotype of the original plant having orange, spotted flowers is OY Ss, showing incomplete dominance between orange and yellow alleles.

Key concepts

Punnett Square

The Punnett Square is a basic but powerful tool in genetics to predict the genotypes of offspring from parental genotypes. It helps visualize how alleles from each parent combine during fertilization. For our plant case, we cross the genotype OY Ss with itself.
To create a Punnett Square, draw a grid and list the alleles of one parent on the top and the other parent on the side. Each cell in the grid shows a possible genotype for the offspring. This visual representation helps us calculate the probability of each phenotype, making it clear which traits will be dominant. This process helped us deduce the potential flower colors and spotting patterns for our plants.

Allele Dominance

Allele dominance is a fundamental concept in genetics. It explains which allele will be expressed in the phenotype when two different alleles are present.
In our plant example, the flower spotting shows a dominant-recessive relationship with the alleles 'S' for spotting (dominant) and 's' for no spots (recessive). When both 'S' and 's' alleles are present as (Ss), the 'S' masks the effect of 's', resulting in a spotted flower.

• A dominant allele: Expressed even if one copy is present (e.g., S in our plant).
• A recessive allele: Expressed only when two copies are present (e.g., ss for no spots).

Knowledge of allele dominance is crucial as it directly impacts the expected phenotypes of the progeny.

Incomplete Dominance

Incomplete dominance is when neither allele is completely dominant over the other. In such cases, heterozygous individuals display a blend or mixture of both parental traits.
For our plant's flower color, incomplete dominance occurs between orange (O) and yellow (Y) alleles. A plant with genotype OY manifests a mix of the two parental colors, resulting in the orange hue we observe. This blending highlights how different genetic mechanisms explain the variation we see in nature.

• Heterozygous examples: Incomplete dominance leads to a new phenotype different from either parent.
• A visual blend provides evidence of mixed allele expression.

Understanding incomplete dominance provides depth to genetic studies, illuminating how traits can mix and vary.

Phenotype Ratios

Phenotype ratios give a numerical glimpse into the diversity of offspring traits. They result from the different allele combinations predicted using a Punnett Square and are essential for understanding genetic predictions.
In our plant scenario, we noted a 3:1 spotted to unspotted ratio, suggesting dominance, and a 10:7 orange to yellow color ratio indicating incomplete dominance.

• The 3:1 ratio shows that a simple dominance relationship is at play for spots.
• The 10:7 ratio suggests that color inheritance doesn't follow traditional dominance, pointing towards incomplete dominance.

Using phenotype ratios helps predict the likelihood of certain traits and deduce the underlying genetic principles governing them.